Organic light emitting diodes (OLEDs) can utilize organic small molecules or polymers that produce light when transferred into their excited state by an external power source. Accordingly, OLED devices may be referred to as polymer light emitting diode (PLED) devices or small molecule organic light emitting diode (SMOLED) devices depending on their active compositions. Depending on the driven mechanisms of the OLEDs, sometimes the terminologies of active matrix OLED (AMOLED) and passive matrix OLED (PMOLED) are used.
Earlier OLEDs were typically based on relatively simple structures, where a thin layer of the electroluminescence (EL) conjugated polymer was enclosed between a pair of electrodes. When a voltage is applied to the electrodes, the positive (anode) and the negative (cathode) electrodes can provide injection of holes and electrons, respectively, into the EL polymer. In the EL polymer layer, electrons and holes move towards each other in the applied electrical field and form excitons, which are bound excited states that can relax down into the ground state radiatively by emitting a photon. This process can be referred to as electroluminescence. OLED devices are of interest in, for example, display, signage, and lighting.
OLEDs were first designed in the 1980s, see, e.g., C. W. Tang, S. A. Van Slyke, Organic electroluminescent diodes, Appl. Phys. Lett. 1987, 51, 913. More recent developments in OLED materials and applications are generally described in Kraft et al., Angew. Chem. Int. Ed., 1998, 37, 402-428, and Z., Li and H. Meng, Organic Light-Emitting Materials and Devices (Optical Science and Engineering Series), CRC Taylor & Francis (Sep. 12, 2006). The disclosures of these references are incorporated by reference in their entirety.